The invention concerns a process for the working-up of salvage oil, in which the de-watered salvage oil is subjected to super-critical conditions using a solvent that is gaseous at normal conditions and subsequently, from the separated super-critical gas phase, the extracted components are deposited by means of lowering of pressure and/or alteration of temperature.
About fifty percent of all technically-utilized lubricating oils originate as impure salvage oil and can, insofar as they are accessible for collection, be introduced to a further utilization. This can occur either through combustion for the purpose of energy recovery or by means of raffination of the salvage oil for the purpose of employment anew as lubricating agent. The salvage oil to be worked up contains herewith impurities of the most various types, such as, e.g., so-called additives (metal compounds serving therefor), wear-and-tear residue and aging residue, as well as impurities which originate from improper storage of the salvage oil. These impurities of the most various types require on the one hand in connection with combustion of the salvage oil, particular measures for protection of the environment and must, on the other hand, be completely separated from the arising product that is to be re-employed, in the context of the raffination of the salvage oil.
The sulphuric acid/bleaching earth technique for raffination of the salvage oil has found particularly broad employment until now. The advantages of such a simple technique are counterbalanced, however, by slight yields, insufficient selectivity and greater amounts of problematical waste products, in the substantial acid resin and bleaching earth residue. Moreover, this technique does not guarantee the harmless elimination of particularly problematical injurious substances such as polychlorianted biphenyls, polychlorinated phenyls, halogen alkanes and polycondensed aromates.
More recent developments in the techniques for salvage oil raffination include:
the IFP-technique (substitution of sulphuric raffination by a super-critical propane extraction),
the BERC-technique (solvent extraction, vacuum fractionation and bleaching earth or H.sub.2 -after treatment),
the KTI-technique (vacuum-thin layer-distillation as purification stage),
the PROP-technique (employment of ammonium phosphates for purification and H.sub.2 -after treatment), and
the recycling technique (separation of the salvage oil impurities by means of sodium).
These techniques, with the exception of the KTI-technique, have not been able to furnish evidence of their essential process-technical qualification and economical technology in large-scale operations.
The KTI-technique with a thin layer evaporator as physical purification stage has indeed been employed on a large scale. The problem of the thin layer evaporation however, consists in the fouling of the heat-exchanger surfaces and the comparatively low selectivity of the technique.
It is already known, moreover, from German Offenlegungsschrifften Nos. 28 50 540 and 30 38 728 to undertake the working up of salvage oil by means of employment of an extraction under super-critical conditions, i.e. temperatures above T.sub.K and pressures about P.sub.K. The manner of operation described in these publications, however, promotes no solution for the harmless elimination of the halogen compounds contained in the salvage oil. Considering today's requirements with regard to environmental protection, an employment of the super-critical extraction for working-up of salvage oil is only possible in practice, when this problem can be solved in satisfactory manner.